Generally, it is necessary that an optical function of an optical element is to be enhanced by, for example, eliminating an unnecessary reflecting light, scattering light or the like, or alternatively, preventing a light from reflecting or scattering thereby efficiently using only an effective light. Various methods have been proposed for such an enhancement of the optical function.
Conventionally, adopted is a method for forming a thin film of SiO2, TiO2 or the like on an optical functional surface of an optical element by using a method such as, for example, vapor deposition, sputtering or coating, so as to prevent an entrance of an unnecessary reflecting light or scattering light. However, a complicated process is required for forming such a thin film, and thereby leading to inefficiency in productivity and hence causing increase in cost. Moreover, such a thin film has a large wavelength dependency, and therefore, an effect of preventing reflection-scattering of a light having a wavelength other than a predetermined wavelength is small. Consequently, over an entire range of visible ray frequently used for an optical element such as an image sensor, an unnecessary reflecting light or scattering light cannot be sufficiently prevented from entering.
As described above, when using, for example, an image sensor, one of critical problems is that an unnecessary reflecting light or scattering light, particularly, a scattering light or a re-reflecting light due to a surface of a micro lens and an inner face of a cover glass, becomes a cause of noise. However, an effective means for solving the problem has not been found.
Further, other than the noise due to the unnecessary reflecting light or scattering light, the following problem is possibly arisen for the image sensor, for example. Though depending on the size of an element or the number of pixels, an area (aperture) attributed for a photo-electric conversion of a light receiving section in an image sensor is usually limited to about 20 to 40% of the entire area. A small aperture leads to reduction insensitivity in the image sensor. Therefore, in order to compensate the sensitivity reduction due to the small aperture, a micro lens for collecting light is generally formed on the light receiving section. A demand for a high definition image sensor whose capability exceeding, e.g., two million pixels, is recently ever increasing. However, the sensitivity reduction due to the decrease in numerical aperture is caused even with the micro lens formed in such a high definition image sensor. Moreover, in such a high definition image sensor also, increase in noise such as a flare or a ghost image due to an unnecessary reflecting light or scattering light is certainly a critical problem in addition to the sensitivity reduction.
In order to solve the problems such as the decrease in numerical aperture and the increase in noise such as a flare or a ghost image, an image sensor such as the following is proposed (see patent document 1).
FIG. 10 shows a schematic structure of a conventionally proposed image sensor. As the schematic cross sectional illustration of FIG. 10 shows, an image sensor 100 includes a main image sensor 80 and a micro lens 90, the main image sensor 80 including photo-electric conversion elements 82, light-shielding sections 83, a planarized layer 84a, color filters 85, a planarized layer 84b and an undercoat layer 86 successively formed on a semiconductor substrate 81, and the micro lens 90 including a resin lens 91 and a porous layer 92 formed on a surface of the resin lens 91, formed on the undercoat layer 86. Here, through subjecting a transparent resin layer which is formed by coating the resin lens 91 with a transparent resin to dry etching, the porous layer 92 is so formed that a void and a thickness of a resin surface thereof is about a quarter of a light wavelength. By forming the porous layer 92 on the resin lens 91, an appearing refractive index at the porous layer 92 is reduced, whereby it is possible to provide an effect of preventing reflection-scattering to the micro lens 90. Accordingly, the problem of the increase in noise such as a flare or a ghost image in an image sensor can be solved to some extent.
[patent document 1] Japanese Laid-Open Patent Publication No. 2002-261261